1. Field of the Invention
This invention relates to a scanning optical system in which pitch irregularities of scanning lines are eliminated.
2. Description of the Prior Art
Heretofore, various light beam scanning systems using a deflecting-reflecting surface such as rotatable polygon mirror have been proposed in which even if the direction of travel of the light beam deflected and scanned is varied in a plane perpendicular to the deflecting surface by the fall-down of the deflecting-reflecting surface, there occurs no irregularity in the pitch of the scanning lines on a scanned surface (a scanned medium). Herein, the deflecting surface refers to the light beam surface formed with lapse of time by the light beam deflected by the deflecting-reflecting surface of the deflector.
For example, in U.S. Pat. No. 3,750,189, the optical system between the deflector and the scanned medium comprises beam shaping means and second converging means, and the light beam reflected by the deflecting mirror is collimated by the beam shaping means. If the optical system is endowed with such collimating function, the shape of the beam shaping means is restricted, so that the degree of freedom which improves the imaging performance on the scanned surface and the strain characteristic for rendering the scanning speed constant is decreased and necessarily, a good performance cannot be obtained unless the number of lenses forming the second converging means is increased.
In U.S. Pat. No. 3,865,465, a certain predetermined limitation is imposed on the ratio of the focal lengths of two different lenses forming the optical system between the deflector and the scanned medium, and satisfying this limitation is equivalent to collimating the light beam in a cross-section perpendicular to the deflecting surface between the two different lenses. Accordingly, again in this example, the degree of freedom with which the imaging performance and strain characteristic are well corrected is decreased as previously described, and this is not preferable.
In U.S. Pat. No. 3,946,150, a cylindrical lens is disposed between a lens having a strain characteristic for realizing equal speed scanning and the scanned medium. In such a construction, a good image cannot be obtained unless the position of the cylindrical lens is brought close to the scanned medium. If the cylindrical lens comes near the scanned medium, the cylindrical lens becomes longer in the direction of the bus line as the scanning width becomes longer and thus, a compact construction cannot be realized.
U.S. Pat. No. 4,123,135 relates to an optical system for correcting the fall-down of the deflecting-reflecting surface of the deflector, and this system is characterized in that a cylindrical lens having an axis perpendicular to the deflecting surface and having a negative refractive power in the deflecting surface but having no refractive power in a cross-section perpendicular to the deflecting surface is disposed in the imaging optical system between the deflector and the scanned medium. This imaging optical system comprises three lenses.